This invention relates to ink delivery techniques for ink-jet printing.
A capillary material such as polyurethane foam is commonly used to maintain backpressure in ink-jet pens. Although this material works well for this purpose, it tends to limit the performance capability of the ink delivery system. During printing, ink is extracted from the foam and the backpressure in the pen increases. The rate at which the backpressure increases depends on the rate of extraction. Print quality suffers if the backpressure increases too quickly, so the allowable ink flux through a foam-based ink delivery system is inherently limited.
Another disadvantage inherent to foam is extraction efficiency. Limiting the ink flux through a foam-based ink-delivery-system will control the rate at which the backpressure increases, but it will not stop the magnitude of the backpressure from increasing. If the backpressure magnitude gets too high, the nozzles will deprime and the pen will fail to print. Unfortunately, the maximum allowable backpressure is reached before all of the ink is extracted from the foam. Foam-based ink delivery systems have been implemented as disposable pens and on-axis replaceable ink supplies, but the inefficiency of both systems increases the cost per printed page. Additionally, when a foam-based replaceable ink supply is separated from the pen, nozzle backpressure and environmental compliance is lost. In this state, light impact or environmental changes may cause the pen to drool. Regulators and spring bags are used to maintain backpressure and provide environmental compliance in ink-jet pens, but these systems result in higher direct material cost and increased manufacturing complexity. Also, these systems are sealed and will eventually become full of air, resulting in pen failure.
An inkjet printing system is described, which includes a printhead having a plurality of ink ejection elements, a first ink supply having a capillary material disposed therein for holding a volume of ink and fluidically coupled with the printhead, and a second ink supply having a volume of ink and fluidically coupled with the printhead. The second ink supply provides ink to the printhead when a differential pressure between the printhead and the second supply exceeds a predetermined pressure.